Technical Field
This Patent Disclosure relates generally to inductive sensing, and more particularly to resonant inductive sensing.
Related Art
Inductive sensors are used to detect/measure events/conditions based on changes in a sensing B-field. The inductive sensor includes a sense (inductor) coil, coupled to an inductance-to-data converter (IDC). The IDC drives the sense coil to project the sensing B-field, and acquires sensor measurements (readout) through the sense coil corresponding to changes in the projected B-field caused by a sensed event/condition.
For example, inductive proximity sensor/switches detect the proximity of a conductive target to the inductive sensor, within a defined proximity switching threshold. Inductive proximity sensors/switches have sufficient dynamic range to detect proximity with nano-meter resolution. However, switching accuracy is limited by temperature drift and component tolerances.
Inductive sensing, including inductive proximity sensing, can be implemented based on resonant sensing in which the inductive sensor includes a sensor resonator with an inductor coil and a series/parallel capacitor (LC tank circuit), with losses in the sensor resonator characterized by a series resistance Rs (loss factor). The IDC drives the sensor resonator to maintain a resonance state (sustained oscillation), projecting a sensing B-field, and acquires sensor measurements corresponding to sensor response to a proximate target as reflected in changes in the resonance state of the sensor resonator.
Sensor resonator response manifested as changes in resonance state can be based on either: (a) measuring changes in sensor resonator losses due to eddy current losses in the target (eddy current sensing), manifested as an increase in sensor resonator impedance, or (b) measuring a change in sense coil inductance due to eddy current back emf, manifested as a change in sensor resonator oscillation frequency. In the case of resonator losses, the resonator loss factor Rs can be characterized by an equivalent parallel impedance Rp (Rp=(1/Rs)*(L/C)), which takes into account frequency-dependent LC reactive impedance, so that changes in total resonator impedance 1/Rp can be measured as a change in the negative impedance −1/Rp required to counterbalance resonator impedance and maintain resonance (sustained oscillation). In the case of resonator inductance, back emf caused by the induced eddy currents effectively changes sensor coil inductance, manifested as a corresponding change in resonator oscillation frequency required to maintain resonance (sustained oscillation). Design considerations include required sensitivity and tolerance for temperature effect: eddy current sensing based on sensor resonator losses is more sensitive, but sensor inductance is less susceptible to temperature effects.
For example, for two identical sensor resonators used in a multi-channel configuration with a single IDC, even if resonator capacitor mismatch is reduced to 0.1% for both LC tanks, the resulting distance error can be 1% of coil diameter.
While this Background information references inductive proximity sensing, the Disclosure in this Patent Document is more generally directed to inductive sensing for applications other than proximity sensing.